kvserver: write new GCHint fields under cluster setting

Since it is late to enable this behaviour in 23.1 (risk of backwards
incompatibility), hide it behind a default-off cluster setting. In 23.2,
it will be enabled by default, and the cluster setting will be
deprecated.

The new GCHint behaviour is likely backwards compatible, but we are
hiding it behind a setting for extra safety.

The safest moment to enable this cluster setting is when there is some
confidence that the cluster binaries will not rollback to previous patch
versions of 23.1. The risk exists only in mixed-version state in which
some 23.1 binaries don't know the new GCHint fields, and some do.

Epic: none
Release note (ops change): introduce a default-off cluster setting
`kv.gc.sticky_hint.enabled` which helps expediting garbage collection
after range deletions, such as when a SQL table or index is dropped.

pkg/kv/kvserver/batcheval/cmd_delete_range.go

@@ -21,12 +21,22 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/lockspanset"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/spanset"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
+	"github.com/cockroachdb/cockroach/pkg/settings"
 	"github.com/cockroachdb/cockroach/pkg/storage"
 	"github.com/cockroachdb/cockroach/pkg/storage/enginepb"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/errors"
 )
 
+// enableStickyGCHint controls whether the sticky GCHint is enabled.
+var enableStickyGCHint = settings.RegisterBoolSetting(
+	settings.SystemOnly,
+	"kv.gc.sticky_hint.enabled",
+	"enable writing sticky GC hints which expedite garbage collection after schema changes"+
+		" (ignored and assumed 'true' in 23.2)",
+	false,
+)
+
 func init() {
 	RegisterReadWriteCommand(kvpb.DeleteRange, declareKeysDeleteRange, DeleteRange)
 }
@@ -135,8 +145,11 @@ func DeleteRange(
 				return err
 			}
 
-			// Add the timestamp to GCHint to guarantee that GC eventually clears it.
-			updated := hint.ScheduleGCFor(h.Timestamp)
+			updated := false
+			if enableStickyGCHint.Get(&cArgs.EvalCtx.ClusterSettings().SV) {
+				// Add the timestamp to GCHint to guarantee that GC eventually clears it.
+				updated = hint.ScheduleGCFor(h.Timestamp)
+			}
 			// If the range tombstone covers the whole Range key span, update the
 			// corresponding timestamp in GCHint to enable ClearRange optimization.
 			if args.Key.Equal(desc.StartKey.AsRawKey()) && args.EndKey.Equal(desc.EndKey.AsRawKey()) {